Concentrometer



A. GOETZ CONCENTROMETER Feb. 3, 1959 Filed Aug. 3o. 1954 5 Sheets-Sheet 2 soo.

0 1 l sa INVENTOR. 44am/swf@ $0572' Fra IJICIIIIY Feb. 3, 1959 A, GoETz 2,871,695

' CONCENTROMETER Filed Aug. so, 1954 3 sheets-sheet s TO VACUUM pump FROM Llqulo SUPPLY INVENTOR. ,4Q 74A/05e @9572 BY L m nited States atent O `CGNCENTROMETER Alexander Goetz, Altadena, Calif.

Application August 30, 1954, Serial No. 453,066

23 Claims. (Cl. 73-61) My invention relates to concentrometers, more particularly to means and method for determining the concentration of particulate matter in a volume of a huid, either liquid or gaseous in nature.

Included in the objects of my invention are:

First, to ,provide a means and method of this class whereby a wedge-shaped or cuneal deposit of particles is collected by retention on the surface of a microporous filter, the deposit varying in thickness or density in vertical direction of same in accordance with a selected mathematically defined function, such as a logarithmic function.

Second, to provide a means and method of this class which may beemployed to collect living as well as nonliving matter;'that is, bacteria, yeast, fungi, or other microorganisms, or non-living particles.

Third, to provide a means and method of this class which by reason of the deposit formation on the surface of a microilter increases in thickness or density, in accordance with a predetermined mathematical function, rendering possible quantitative determination of the concentration of the particles even though the concentration thereof in the fluid medium may embrace an extremely wide range.

Fourth, to provide a means and method of this class wherein the concentration of the particles under investigation may be determined by a differential method wherein a selected horizontal section of the membrane deposit is compared with a standard or known sample colorirnetrically or photometrically, or by other observational methods.

Fifth, to provide a means and method of this class wherein the concentration of the particles, specifically microorganisms which can be caused, in contact with nutrient matter, to form discrete countable colonies, may be determined by an integrating method wherein the area beginning at the upper edge of the microfilter which carries a preselected number of colonies is measured on a scale affixed, or otherwise correlated, to the vertical dimension of the filter.

Sixth, to provide a means and method of this type which may be employed to determine the inhibitory potential of antibiotics for specific bacterial strains, or in general of substances capable of acting on or reacting with the particles deposited on the microfilter.

Seventh, to provide a novel microfilter membrane across which may be established a pressure differential to pass a liquid therethrough, but which, when wetted, partially causes that portion of the filter exposed above the liquid level to become impervious to air so 'that` the pressure differential required vfor the passage of liquid through the portion of the filter below the liquid level may be maintained.

Eighth, to provide a microfilter membrane which, while dry, will pass air or gas., but may be rendered impermeable by a liquid gradually covering and wetting one side thereof, thus to effect a progressive change in the effective lter area.

2,871,695 Patented Feb. s, 1959 Ninth, to provide a means and method of determining the concentration of particles in a liquid sample wherein the filter membrane forms one wall of a cavity containing the sample and where means are provided to change the level of the liquid in the cavity so as to progressively uncover or cover the microfilter membrane and correspondingly control the fraction of the area through which the sample uid can pass.

Tenth, to provide a means and method of this class which, though primarily directed to the concentrometric determination of particles suspended in a fluid medium, may be employed to determine the concentration of -molecules or ions dissolved in the sample fluid.

Eleventh, to provide as an article of manufacture a microlter membrane having a deposit thereon, which progressively increases in thickness or density in accordance with a predetermined distribution function capable of mathematical definition.

With the above and other objects in View, as may appear hereinafter, reference is directed to the accompanying drawings, in which:

Figure l is a plan view of my concentrometer;

Fig. 2 is a sectional View thereof through 2-2 of Fig. l, showing in addition and substantially diagrammatically the means for filling the concentrometer with a sample solution and withdrawing the solution from the concentrometer;

Fig. 3 is an end view of my concentrometer with the filter carrier gate in open position, and taken from the plane 3--3 of Fig. 1;

Fig. 4 is a diagrammatical perspective View, illustrating a geometric figure defined by that portion of the concentrorneter cavity confronting the microlter membrane;

Fig. 5 is a fragmentary view of a microfilter membrane, indicating diagrammatically an uncountable type of deposit'thereon;

Fig. 6 is a similar fragmentary view of the microfilter membrane, illustrating diagrammatically a countable type of deposit of discrete particles thereon;

Fig. 7 is a diagrammatical sectional view through 7-7 of Fig. 5, showing the uncountable type of deposit, the thickness of the microfilter membrane and deposit being greatly exaggerated, While the scale of the variation of the particle density thereon is greatly diminished;

Fig. 8 is a sectional View through 8-8 of Fig. 6, showing a countable type of deposit, the thickness of the microfilter membrane and deposit being greatlyvexaggerated;

Fig. 9 is a reduced elevational view of a microfilter membrane, indicating the area in which a latent deposit of uncountable microorganisms occur;

Fig. l0 is a reduced elevational view of a microfilter membrane which has been impregnated with or on which has been imprinted a series of inhibitory substances, such as antibiotics;

Fig. 1l is a reduced elevational view of the microfilter membrane shown in Fig. 9 upon culture of the microorganisms as affected by the antibiotics represented in Fig. 10;

Fig. l2 is a fragmentary sectional view of the microfilter membrane, shown in Figs. 9 and l0, and an underlying nutrient pad, the thickness thereof being greatly exaggerated;

Fig. 13 is a plan view of the means employed for eountable deposits to determine the concentration of the microorganisms represented thereby, wherein an integrating method of determination is used;

Fig. 14 is a similar plan view of a modified device for use particularly with uncountable deposits, whefein a differential method of determination is employed;

Fig. l5 is an elevational view of a microfilter memase/m95 brane having a typical cuneal deposit thereon, in either latent or apparent condition;

Fig. 16 is a similar elevational View, showing another deposit with the microlter membraneV turned at 90 to Fig. l;

Fig. 17 is an elevational view, showing the membranes of Figs. l5 and 16 in superposed relation and indicating typical interaction between the deposits on Figs. and 16; and

Fig. 18 is a fragmentary longitudinal sectional View similar to Fig. 2, illustrating a modified form of my concentrometer employed in the determination of the concentration of microparticles in air.

Reference is first directed to Figs. l, 2, and 3. My concentrorneter here illustrated comprises a body structure li which, for reasons that will be brought out hereinafter, is substantially triangular in plan. The body structure l. is provided with a sloping bottom wall 2 and a vertical front wall .3 at the truncated apex of the triangle. The front wall 3 is provided at its upper portion with an aperture 4. Side walls 5' diverge from the front wall vand are joined to a rear end wall d, forming the base of the trapezoidal body structure. .he body structure is mounted on a pedestal 7 which in turn is supported by a tripod 3, preferably arranged with adjustment screws at its extremities. The `body structure is equipped with levels 9 so that the body structure may be vappropriately leveled.

The front wall 3 is covered by a gate l@ suported at one vertical end by a hinge 11. The opposite end of the gate is adapted to be secured to the wall 3 by suitable latches 1 2. The gate lil is provided with a porous section 13 coextensive with the aperture 4. The porous section may -be formed ofV porous ceramic material, graphite, wire mesh, or the like. lts purpose is to form a support for a microlter membrane 14 adapted to be clamped between the front wall 3 and the gate lll. The properties of the microfilter membrane will be set forth in more detail hereinafter.

Formed in the gate l0, behind the porous section i3, is a cavity 15 communicating at its lower end with a drain passage 16. A ller passage ll traverses the drain passage and communicates with the interior of the body structure l immediately above the bottom wall 2. The passages 16 and 17 are interrupted by a rotatable valve element l having a diametrical port so that either the drain passage or the filler passage may be opened and the other passage closed.

The drain passage 16 communicates with a drain tube 19 which leads to and is sealed in the upper end of a collector vessel Ztl. The collector vessel is adapted to receive liquid drawn through the microlilter membrane i4 and porous section 13. For this purpose the collector vessel 2l) is provided with a vacuum outlet 21 connected to a source of reduced pressure. The collector vessel is of sufficient size to receive all of the liquid delivered through the microllter membrane and is preferably provided with suitable graduations.

The ller passage 17 is connected to a filler tube 22 which communicates with a sample vessel 23. The sample vessel 23 is adapted to be raised so that its contents may be delivered to the body structure l and reach a level coincident with the upper extremity of the porous section 13.

The body structure 1 is covered by a contoured plate 2d which is sealed to the side walls 5 of the body member and to the bottom wall 2. a short distance forward of the back wall 6. Between the contoured plate 2d and back wall 6 there is formed a sump 25 having a discharge port. Between the forward edge of the contoured plate 24 and the upper edge of the porous section 13 there is formed a bleed slit 26.

To facilitate handling of the microflter membrane, mating covers of the gate 10 and wall 3 are notched, as indicated by 27.

The region within the body srtucture 1, between a liquid level indicated by L, which is coincident with the lower edge of the porous section and the contoured plate 24, denes a mathematical figure. That is, the area in succeeding horizontal planes of this ligure, beginning at the upper edge of the porous section 13 and terminating at the lower edge thereof, that is, at the level L, varies according to a selected mathematical function. The body is made triangular in plan, otherwise the plate 2.45 would be unduly long.

ln order for the concentrometer to function eiiciently, the microlilter membrane should be, of a type wherein microparticles are deposited on the surface rather than within the interstices of the microlter membrane. Also, the ditfusivity of the microlter membrane perpendicular to its surface should be much greater than in a direction parallel to the surfaces.

rl`he pores in the microlter membrane should be of uui form size, and, of course, smaller than the microparticles to be retained. Preferably, these pores are smallest at the surface of the membrane facing the liquid, that is, toward the cavity within the body structure l. These conditions are met by a membrane which consists of porous xero-(dry) gels of miscellic structure, the elements of which are of molecular orderv of magnitude and are held permanently by intermolecular forces without the use of binding agents. The material is thus characterized by a complete absence of fibrous structure.

An extremely fine mesh is formed by the loose network of cross-linked particles of a balance combination of cellulose ester polymers. Such material available on the market has the appearance of tine, thin, very white paper (about 150g. thick or 0.006 inch) with a semi-glossy surface of extremely low specific gravity (.3-.5) duc to a porosity of between -90%. Chemically, the material consists of incompletely cross-linked high polymer molecules of partially substituted cellulose nitrate and acetate. The macromolecules consist of chains of ZOO-500 glucose units equivalent to a straight molecular chain length of the order of lil-L104 cm.

Microscopically, the small size of the structural elements of the filter reveals no detail within `the resolving power of the optical microscope. The pore size and configuration of the molecular filter material is determined by an involved interaction of a physical and chemical nature between the solvated molecules of the constituent cellulose esters during several controlled gelation steps resulting in a xero (dry)gel. rlhis xero-gel constitutes the microiilter membrane. lt contains only the dry cellulose esters and is stable in both the dry and wet state. The effective pore size of a microlter membrane may, by control of the manufacturing procedure, bc varied and over a wide range, approximately l-5000/i.

While microlters of the described type are eminently suitable for this purpose, l do not wish to restrict the method to the use of such specific materials. ln fact, any porous material may `be used, if it fulfills two conditions: namely, that its pore size is very uniform over the entire area, and that the pore size is so small thatthe differential pressure required to displace from the pore a liquid of the surface tension of the sample liquid is larger than the differential pressure applied for filtra tion.

The pore size determines the particle size range of thc substances which can be retained and the concentration of which is to be determined. In any case, the particle size should be small enough that during the filtering period there is no appreciable settling of the particles out of solution; that is, the settling rate as determined by Stokes law is small compared to the ovv rate of the liquid through the microfilter membrane.

Operation of my concentrometer is as follows:

The liquid containing the substance, the concentration of which is to be measured, is delivered from the sample vessel 23 to the body structure 1 through the passage 1.7

4untilpa portion ofV the sample liows out the bleed slit 26. This portion may be collected in the sump 25. The valve 18 is then turned to the position shown in Fig. 2L The vacuum pressure existing in the collector vessel 23 creates the pressure differential across the microilter membrane 14, causing the liquid to fiow therethrough.

It will be observed that, initially, the entire area of the microlter membrane exposed through the aperture 4 serves as a filter, but as the liquid level is lowered, the effective filtering area of the microfilter membrane is correspondingly reduced. Also, it will be observed that the quantity of liquid which passes through the upper horizontal portions of the microfilter membrane will be much less than the quantity of liquid which passes through the lower horizontal portions of the microfilter membrane. The relative quantities of liquid will depend upon the horizontal cross-section of the cavity as represented diagrammatically in Fig. 4. By proper shaping of this be pointed out that the microfilter membrane is provided with suitable index or reference marks (not shown) which enable it to be placed in exactly corresponding position in the concentrometer as well as the counting device. The plate 32 and one of the guide flanges 31 are provided with a scale 33 corresponding exactly to the distribution function of the cavity shape.

The plate 32 is moved so that its upper edge 34 uncovers a predetermined number of colonies, for example, 1t). Thus, in the example, shown in Fig. 13, if the scale represents milliliters and indicates twenty, this would mean that the statistical average concentration amounts to fifty organisms per one hundred milliliters. And it may be noted that the smaller the predetermined number, the larger the statistical uncertainty; also that different i cencenrations of different samples determined by this cavity the thickness or density of the deposit on the surface of the microiilter membrane may follow a large variety of distribution functions, the choice of which is determined by the nature of the concentrometric information required, such as linear, exponential, hyperbolic, and logarithmic functions.

The microparticles may be living or non-living matter, that is, the microparticles may constitute bacteria, yeast, fungi, or other microorganisms, or may be nonliving matter suspended in the sample fluid. If the particle be a living microorganism, the deposit on the microfilter membrane may be virtually invisible and might be defined as a latent deposit. In such case, the deposit is incubated in contact with a nutrient in a conventional manner, or in a manner more fully disclosed in my copending application, Serial No.,332,288, filed January 2l, 1953, now Patent No. 2,761,813. It should be observed that there are literally hundreds of nutrient compositions or inhibitors for the selective growth promotion of a multitude of species of bacteria, yeast, fungi, etc., all well known in the iield of microbiology.

The deposit variation on the microfilter membrane is made to vary in density in accordance with a logarithmic function. It follows that the number of microorganisms collected at the lower edge of the microilter membrane may be a thousand or many thousands times greater than that collected at the upper edge. Thus theconcentration of the microorganisms in the sample liquid may vary over a very extended range, and yet be susceptible of uniform relative accuracy of determination.

Assuming, for example, that my concentrometer is employed for the determination of the concentration of bacteria in a water sample, the latent deposit of bacteria is developed by causing the bacteria to grow into colonies of discernible size and then by inhibiting the bacteria growth in the conventional manner. The developed bacterial deposit may then appear as shown in Fig. 6, in which case the number of bacterial -colonies will increase, in accordance with the predetermined distribution function, from a starting point at the upper horizontal edge of the microiilter membrane downward, until the number of colonies is so numerous as to be uncountable.

By selecting some statistical standard number of colonies, for example ten, and determining the horizontal border above which these ten colonies appear, the concentration of the bacteria may be determined. This may be termed integrating method of determining the bacterial concentration. This may be accomplished by a scale which may be imprinted at one vertical margin of the microfilter membrane, and corresponding scale lines may extend across the membrane. A more accurate arrangement, however, involves the use of a device 29, as shown in Fig. 1

The device 29 comprises a frame 3i? having guide flanges 31 at either side between which is slidable a plate 32. The distance between the guide fianges is made to exactly locate the microlter membrane therein. It should method and using equal numbers have equal statistical weight.

A further example, involving not only the determination of the concentration of microorganisms, but also the relative efficacy of various inhibitors or antibiotics, may be accomplished by the method represented in Figs. 9 through 12. More specifically, the latent deposit of microorganisms, as represented by the microlter membrane in Fig. 9, is placed on a second microlter membrane 41, shown in Fig. 10, which has been impregnated in any suitable pattern, for example, in parallel strips, with several antibiotics, I, Il, IH, IV. One strip may be left free to represent a control C. It should be pointed out that the microfilter membrane may be readily impregnated with a chemical in solution and later dried without appreciably altering its filtering characteristics.

The microlter membrane 14 with the latent deposit is placed over the membrane 41 containing the antibiotics, and these placed on a nutrient pad 42 of absorbent material. As has been pointed out hereinbefore, a wide range of nutrients are well known in the iiield of bacteriolo-gy. Sufficient moisture is provided to permit the nutrient to migrate upward through the two microfilter membranes and supply the bacteria. The bacterial colonies will only grow where the nutrient, as modified by the addition of the antibiotic, is insufhcient to inhibit growth. After incubation or development of the latent deposit, the microfilter membrane 14 may appear as in Fig. l1 wherein it is indicated that antibiotic I is partially effective, antibiotic II particularly effective, antibiotic III is partially effective, and antibiotic IV has virtually no effect. If the scale follows a logarithmic function, it can be seen that the quantitative efficacy of a variety of antibiotics over a range of several thousand may be determined by one single experiment.

While the examples given have been related to microorganisms or living microparticles, it should be observed that my concentrometer is not limited to microorganisms but may be employed in determining the concentration `of non-living microparticles; that is, any suspended matter of sufciently large particle size as to be retained by the microlter membrane surface and sufficiently small as to remain in suspension during the filtering period.

It is often necessary or desirable that the deposit on the microfilter membrane of an uncountable number of nondiscrete particles be determined. This case is represented in Figs. 5 and '7. In such case a differential device 35, as shown in Fig. 14, is employed.

The differential device 35 includes a frame 36 having guide ilanges 3?, and a plate .3S having a slit 39 of predetermined width. The slit 39 is made as narrow as possible, but must be sufficiently wide so that the underlying deposit may be discerned by the observational method employed. A simple method of using the differentiating device 35 is to compare the color intensity of the portion of the deposit exposed by the slit 39 with a sample.

in not all instances, however, is it necessary for the material to be determined to have a particle size capable of retention on the surface of the microtilter membrane,

as the same basic principle may be used to measure the concentration of dissolved substances. For this application of the method the membrane has to be conditioned in such a manner as to sorb, especially chemi-sorb, the type of molecule or ion to be delivered. VAn example, typifying a large possible variety is the following, specilically adapted to measure small concentrations of Silver (Agir) in solution:

Cadmium sulde (CdS) is precipitated in the microfilter membrane. This may be accomplished by impregnating the membrane withina solution of cadmium salt, such as cadmium chloride (CdClz), then precipitating cadmium sulfide in situ by treatment with hydrogen sulfide (HZS). The quantity of a cadmium sulde (CdS) in the filter matrix is, of course, determined by the concentration the initial solution used for impregnation. The microlter membrane will then have a characteristic orange color and will be suitable for the detection of silver ion in a water sample by allowing the solution to liow through the microfilter membrane from the upper to the lower edge with varying flow density.

The silver ions, as they pass through the microlter membrane, react with the cadmium sulfide to form silver sulfide (AgzS) which is characteristically black. A definite line of demarcation will then be discernible on the microfilter membrane, the position of which can be interpreted .in terms of the concentration of silver ion in the Sample by colorimetric comparison with the dierential method described. it will thus be observed that in the measurement of the concentration of molecules or ions, the chemical contained in the membrane matrix must be sulhciently insoluble as to remain within the lter during the filtering process, and the ion to be detected must form a less soluble compound than the reagent contained initially within the microlilter matrix, to eiect a reaction.

My concentrometer may be employed still in another manner whereby reciprocity curves or reaction equilibrium curves or graphs may be obtained virtually automatically, whereas comparable curves would otherwise require a large series of tests in order to locate suiicient points to dene the curve. This is best illustrated in Figs. through 17. A first solution is passed through a microlilter membrane to form a deposit, designated 43, of microparticles suspended therein. A second solution is passed through a second microlilter membrane to produce a second deposit 4d of microparticles suspended therein. Either one or bo-th of these deposits may be living or non-living microparticles. For example, one may be bacteria, the other an antibiotic in a gel so that it may be entrapped on the surface The two microfilter membranes are preferably square so that they may be placed at right angles on each other. The two microiilter membranes are superposed with one membrane rotated ninety degrees relative to the other with the deposit 43 on top, if this be a deposit or" microorganisms, and the antibiotic underneath. These are placed on a nutrient pad, as shown in Fig. 12, and the microorganism 43 permitted to develop or incubate. The result will be that a diagonal line of demarcation will occur between the region of inhibited and uninhibited growth. If the antibiotic and microorganism react for all ranges of concentrations, this will be a straight line X--X, representing a line of constant proportionality. That is, at any point `along such line the ratio of one substance to the other will remain constant. However, if this ratio does not hold true throughout the range of concentrations present, the line will curve as indicated by Y-Z and Y- In the example of bacteria and its antibotic, there may be a concentration of antibiotic beyond which no concentration of bacteria will tolerate, hence the curve would depart towards Y. Similarly, there may be a low concentration of antibiotic which has no eiiicacy whatsoever, in which case the curve will veer towards Y. Similar conditions may occur between two non-living microparticles. It will be observed that in order to dupliof the microlilter membrane.

cate the curve produced automatically by superposing the two microfilter membranes, it would involve a whole series of separate tests employing different concentrations of each of the substances involved. It should be observed that the reaction is obtained by supplying water or other solvent from a suitable pad for transfer through both microlter membranes.

Reference is now directed to Fig. 1S. In the method and apparatus heretofore described, consideration has been given to the determination of the concentration of inici-@particles in a liquid. It is possible to utilize my concentrometer in the determination of the concentration of microparticles present in air or other gases. This is accomplished by means of a body structure 51 having a cavity 52, including a lower wall 53 corresponding in contour to the contoured plate 24. In plan, the body structure is preferably trapezoidal, as in the first described structure. As has been pointed out hereinbefore, the configuration of the body structure is determined by the desired variation of the deposit density to be obtained on the microfilter membrane.

The body structure is provided with a top wall 54 and is completely enclosed except for an aperture 55' at its apex end covered by a porous section S6, which may be in the form of a flne mesh screen or any structure which does not exhibit capillary action in a direction parallel to its surfaces. The porous section or screen 56 supports on its outer side, away from the cavity 52, a microlilter membrane 14. The porous section or screen and microfilter membrane are covered by a gate 57 which may be apertured to expose the area of the microiilter membrane commensurate with the porous section 56, or which may be arranged for connection to the source of air or gas to be investigated.

The body wall 53 of the body structure 51 is provided adjacent the aperture 55 with an intake slot '53 which communicates with a liquid intake passage S9, preferably equipped with-a control valve B. The intake passage 59 communicates with a source of neutral liquid, not shown. The top wall 54 of the body structure is provided with a port 61 for communication with a source of vacuum pressure.

Operation of the construction shown in Fig. 18 is as follows: v

The liquid level is initially at the bottom of the aperture 55. Air or other gas to be sampled is drawn through the microiilter membrane and through the porous wall or screen S6 into the body structure and delivered to the vacuum pump. The vacuum pressure created within the body structure draws in the neutral liquid at a rate dependent upon the setting of the control valve 6l?. The rate at which the liquid level rises depends upon the conliguration of the bottom or lower wall 53 of the body structure. The rising liquid progressively seals oft the microiilter membrane, reducing its effective capacity.

It will be observed that only a small quantity of air is drawn through the lower portion of the microtilter membrane and that the quantity of air drawn through progressively increases toward the upper margin. As indicated in connection with the first described structure, this may be a predetermined logarithmic increase or an increase in accordance with any other desired mathematical function. For convenience, of course, the vacuum pump may be `operated at a constant rate. It will be observed that the scale factor may be altered by adjustmeut of the control valve 60.

It should be noted that the microlter membrane has relatively high dielectric properties which have the eiect of entrapping, on the surface of the microfilter membrane, particles which are actually smaller than the pore space.

While the microlilter membrane has been shown as occupying a vertical position, it may, of course, occupy an angular position, or in fact may be curved in one or two dimensions. The only eiect of this is to change the mathematical function of the deposit which is collected thereon. It should be observed that although the body member is shown as trapezoidal, its sides may be curved vertically or longitudinally in order to obtain the desired variation in density of deposit on the microlter rnem-- brane. Or in other words, any variation in the'horizontal or vertical cross-section may be made in so far as it is instrumental in the generation of the desired variation in the deposit.

It should be observed that even though the deposit, as it increases in density, may restrict the rate of flow, this has no effect on the thickness or density of the l deposit; for the thickness or density is only dependent on the volume, not the rate, of flow of liquid through the filter as long as the total time involved to filter the liquid is not so great as to permit appreciable settling of the microparticles, or appreciable evaporation of the body of the liquid or the liquid sealing the pores of the microfilter membrane against air flow.

It will be observed that my apparatus and method of determining the concentration of substances in a liquid has a wide range of application, including, but not limited to:

(l) Collection of a cuneal deposit of microorganisms, such as bacteria, yeast, fungi, such deposit having a density varying in accordance with a mathematically defined function.

(2) Collection of a cuneal deposit of particles of nonliving, non-discrete substances, such deposit having a density varying in accordance with a mathematically clefined function.

(3) Measuring the concentration of countable particles by an integrating method.

A(4) Measuring the concentration of uncountable particles by differential methods.

(5) Superposing two or more microfilter membranes for interaction between (a) living microorganisms, (b) microorganisms and nutrients, inhibitors, or (c) between non-living particles, wherein: (l) one or more microfilter membranes have cuneal deposits and another membrane is impregnated uniformly with one or more reacting substances, and (2) two or more membranes have cuneal deposits which are caused to interact.

(6) mpregnating the microlter membrane with a test substance which reacts with a filterable substance as the filterable substance passes through.

(7) Fabrication as an article a microfilter membrane with a deposit thereon which varies in density in accordance with a mathematically defined function.

Having fully described my invention, it is to be understood that I do not wish to be limited to the details herein set forth, but my invention is of the full scope of the appended claims.

I claim:

l. An apparatus for determining the concentration of a substance distributed in a quantity of fluid, comprising: means defining a cavity adapted to contain a liquid; means for progressively altering the level of a liquid in said cavity; a microlter disposed so as to be traversed by the surface of said liquid as its level is altered and having a progressively decreasing filter area and a progressively increasing sealed area, one area being below said liquid surface and the other being above said liquid surface; means for establishing a pressure differential across said microfilter to cause flow of fluid through said progressively decreasing filter area, whereby said microfilter retains progressively increasing quantities of said substances; and means for detecting the substance retained by said microlter.

2. An apparatus as set forth in claim l, wherein: the fluid drawn through said microlter is said liquid itself, the liquid level progresses from an upper to a lower margin of said microfilter, and said liquid wets said microfilter to seal against the flow of air through that portion of the microfilter above said liquid level.

3. An apparatus as set forth in claim l, wherein: the fluid drawn through said microlter is gaseous, the liquid level progresses from a lower to an upper margin of said microfllter, and said liquid seals that portion of the microfilter below the surface of said liquid.

4. An apparatus as set forth in claim l wherein: said microlter has a pore size in one surface sufficiently small to retain said substance on said surface thereof, whereby said substance forms a deposit thereon, of progressively varying thickness.

5. An apparatus as set forth in claim l, wherein: said microiilter is impregnated with a test substance capable of reaction with the distributed substance retained by said microfilter, said test substance being included in said detecting means.

6. A method of determining reaction characteristics between substances, characterized by: forming a cuneal deposit of substance on a surface of a microiilter membrane, said deposit varying in density in accordance with a predetermined mathematical function; treating a second microfilter membrane with at least one test substance; placing said mierolter membranes in superposed relation; and causing diffusion of said test substance into said deposit.

7. A method of determining reaction characteristics between substances, characterized by: forming cuneal deposits of substances to be reacted on the surfaces of a pair of microlters, each of said deposits varying in intensity in accordance with a mathematical function in one direction and uniform in a direction at right angles thereto; superposing said mircrolter membranes with said deposits in angular relation; and causing diffusion of one deposit into the other.

8. An apparatus for determining the concentration of a substance distributed in a fluid, comprising: an entrapping means having a multiplicity of parallel passages; means for progressively sealing said passages in accordance with a mathematical function to establish a correspondingly diminishing unsealed area and a complementary increasing sealed area; means for passing a uid through said progressively diminishing unsealed area, whereby the volume of fluid passing through succeeding increments of said area varies in correspondence with said mathematical function; said entrapping means adapted to retain the substance distributed in said fluid also in accordance with said mathematical function; and means for detecting the presence of said substance retained by said entrapping means.

9. An apparatus as set forth in claim 8, wherein: the fluid passed through the diminishing unsealed area of said entrapping means .is a liquid containing said substance and having a surface which progresses from an upper to a lower margin of said entrapping means, and has sufficient surface tension to be retained in said passages as said liquid surface is lowered to constitute said sealing means in the area of said entrapping means above said liquid surface.

l0. Au apparatus as set forth in claim 8, wherein: the fluid drawn through the diminishing unsealed area of said entrapping means is gaseous, said sealing means is a liquid capable of sealing said passages in said entrapping means, and said liquid is caused to raise from a lower to an upper margin of said entrapping means.

ll. An apparatus as set forth in claim 8, wherein: the passages of said entrapping means are smaller than the particle size of said substance whereby said substance forms a deposit on the surface of said entrapping means varying in thickness in correspondence with said mathematical function.

l2. An apparatus as set forth in claim 8, wherein: the passages of said entrapping means are dimensioned to pass the substance distributed in said fluid, and the walls of said passages are coated with a test substance ineluded in said detecting means.

13. An apparatus for determining the concentration of aes/neas 11 asubstance distributed in a Huid, comprising: a microfilter membrane having pores substantially perpendicular to its surfaces and substantially free of connection parallel to its surfaces, whereby said membrane exhibits no appreciable capillary action parallel to its surfaces; means for progressively sealing said pores in accordance with a mathematical function to establish a correspondingly diminishing unsealed ltering area and a complementary increasing sealed non-filtering area; means for passing a uid through said progressively diminishing unsealed filtering area, whereby the volume of uid passing through succeeding increments of said filtering area varies in correspondence with said mathematical function, and the substance contained in said fluid is retained by said microfilter membrane also in accordance with said mathematical function; and means for detecting the presence of said substance retained by said microllter membrane.

14. An apparatus as set forth in claim 13, wherein: the fluid passed through said microlter membrane is a liquid having a surface which progresses from an upper to a lower margin of said microilter membrane, and has sufficient surface tension to be retained in said pores as said liquid surface is lowered to constitute said sealing means in the area of said microlter membrane above said liquid surface.

15. An apparatus as set forth in claim 13, wherein: the huid drawn through said microlter membrane is gaseous, and said sealing means is a liquid caused to progress from a lower to an upper margin of said microlter membrane.

16. An apparatus as set forth in claim 13, wherein: the pores of said microiilter membranes are smaller than the particle size of said substance whereby said substance forms a deposit on the surface of said microtilter membrane varying in thickness in correspondence with said mathematical function.

17. An apparatus as set forth in claim13, wherein: the pores of said microflter membrane are dimensioned to pass the substance distributed in said fluid; and said microlter membrane is impregnated with a test substance included in said detecting means.

18. An apparatus as set forth in claim 8, wherein: said mathematical function is a logarithmic function.

19. An apparatus as set forth in claim 13, wherein: said mathematical function is a logarithmic function.

20. An apparatus as set forth in claim 8, wherein: said means isa liquid container so shaped as to cause the volume of iluid passing through said progressively diminishing area to vary in accordance with a logarithmic function.

21. An apparatus as set forth in claim 13, wherein: said means is a liquid container so shaped as to cause the volume of fluid passing through said progressively diminishing area to vary in accordance with a logarithmic function.

22. A microlter membrane having pores communieating between the surfaces thereof, and substantially free of connection parallel to its surface, whereby said membrane exhibits no appreciable capillary action parallel to its surface; and a relatively insoluble test substance coating the walls of the pores of said membrane, the volumetric density of said test substance varying across said membrane in accordance with a mathematical function.

23. A microilter membrane having pores communieating between the surfaces thereof, and substantially free of connection parallel to its surface, whereby said membrane exhibits no appreciable capillary action parallel to its surface; and a cuneal deposit of microparticles on the surface of said membrane varying in volumetric density in accordance with a mathematical function.

References Cited in the file of this patent UNITED STATES PATENTS 2,353,760 Richards July 18, 1944 2,584,052 Sandorf et al. Ian. 29, 1952 2,672,432 Goetz Mar. 16, 1954 FOREIGN PATENTS 190,143 Great Britain Aug. 23, 1923 

